Comment 10 for bug 870969

So; for the comment #6 - indicating you need to mute one channel but not the other, that's a hardware dependent bug (and it only occurs for the internal mic, no other inputs) and is solvable at least for some Realtek codecs. Please file a separate bug using "ubuntu-bug alsa-base" - doing so will include relevant hardware info.

If there is a generic problem with Skype only (and other apps work fine), I would need a PulseAudio log to do further troubleshooting. See https://wiki.ubuntu.com/PulseAudio/Log